> What happens, if the solder joints of a module touch the cabinet wall

At best, it will probably make the module not function correctly; at
worst it could badly damage the module and/or blow a fuse (or worse) in
the power supply. If you value your module at all, make sure that it
absolutely cannot short anywhere to the case at all! If you really must
place it there, line the inside of the case with something *known* to
be non-conductive, like strips of insulation tape, and be really,
really sure that nothing on the PCB can touch the case. And also make
sure that sharp pins on the PCB cannot penetrate through the tape and
thus short to the case that way.

In general, shorting things on PCBs that aren't meant to be shorted is
definitely *not* a good idea: often you can get away with it (an
oscilloscope probe slipping off and touching the next IC leg is a
common thing to do); sometimes however it can result in instant death
to whatever component gets shorted, necessitating in a tedious hunt for
that component and then replacing it, or further expensive paying
someone else to do this for you, or even simply replacing the module!

The old motto is well heeded: if in doubt, don't!

Tim